This is the quote I've been looking for since I saw the Platformer article about the current cancel campaign against SubStack. Chomsky said this more than 30 years ago in the context of the Faurisson Affair;
https://web.archive.org/web/20071006101318/http://www.chomsky.info/articles/19801011.htm
The more things change, the more they stay the same. It's worth noting Chomsky's point that anti-Nazi pundits are as likely be a Stalinists as they are to be pro-democracy leftists, given the historical amnesia on the contemporary left about its own dark side.